Stacked organic light emitting device having high efficiency and high brightness

ABSTRACT

A stacked organic light emitting device that includes an anode connected to an external power source, a cathode connected to the external power source, at least two light emitting sections aligned between the anode and the cathode, including a light emitting layer, and an internal electrode aligned between the light emitting sections. The internal electrode is a single-layered internal electrode which is made from one selected from the group consisting of a metal, alloys of the metal, and metal oxides thereof, having a work function below 4.5 eV, each light emitting section includes an organic material layer containing an organic material having an electron affinity above 4 eV, and the organic material layer is formed between the light emitting layer of the light emitting section and the electrode facing the anode connected to the external power source in two electrodes which make contact with the light emitting section.

This application claims priority to Korean Application Nos.10-2004-0024470 filed on Apr. 9, 2004 and International PatentApplication No. PCT/KR2005/001001, both of which are incorporated byreference, as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a stacked organic light emittingdevice. More particularly, the present invention relates to a stackedorganic light emitting device including a single-layered internalelectrode aligned between stacked light emitting sections.

BACKGROUND ART

An organic light emitting device is a semiconductor device forconverting electric energy into optical energy. The organic lightemitting device includes two opposite electrodes (an anode and acathode) for applying external power to the organic light emittingdevice and an organic material layer aligned between the anode and thecathode in order to emit light having a wavelength of the visible rayrange when a hole is recombined with an electron. As a forward electricfield is applied to the organic light emitting device having the abovestructure, holes and electrons generated from the anode and cathode areinjected into the organic material layer so that the holes are combinedwith the electrons in the organic material layer, thereby generatingexcitons. When the excitons have become the ground state, light isgenerated. Recently, an organic material layer with a multi-layeredstructure (see, Applied Physics Letters, vol. 51, no. 12, pp. 913-915,1987) has been suggested in order to allow the holes and electronsgenerated from the electrodes of the organic light emitting device to beeffectively injected or transferred into the organic material layer. Ifthe organic material layer is formed with the multi-layered structure,it is possible to drive the organic light emitting device withsignificantly reduced operational voltage while improving light emittingefficiency of the organic light emitting device.

In the meantime, various attempts have been made to realize highbrightness in organic light emitting devices. For instance, a method ofincreasing current density obtained by applying an electric field to theorganic light emitting device has been suggested. However, since anorganic material layer and a thin film structure of the organic lightemitting device are weak against heat, higher current density may exerta bad influence upon the organic material layer and the thin filmstructure of the organic light emitting device, so that the stability ofthe organic light emitting device may be degraded as the current densityincreases. For this reason, current studies are focused on organic lightemitting devices representing high brightness at low current density.

There are two approaches to obtain high brightness at low currentdensity. The first approach is to use an organic material capable ofimproving generation efficiency of excitons, which are generated due torecombination of holes and electrons, and/or generation efficiency ofphotons, which are generated when the excitons have become the groundstate. The second approach is to stack at least two organic lightemitting device units in series, in which each organic light emittingdevice unit includes an anode, a cathode and a light emitting sectionhaving a light emitting layer capable of emitting light by receivingholes and electrons from the anode and the cathode, respectively. In thefollowing description, a term “stacked organic light emitting device”refers to the structure including at least two organic light emittingdevice units stacked in series. The light emitting section may includean organic material layer having a multi-layered structure including ahole injection layer, a hole transport layer, a light emitting layer andan electron transport layer, if necessary.

Methods of fabricating the above-mentioned stacked organic lightemitting devices have been disclosed in various documents.

For instance, International Publication No. WO95/06400 discloses astacked organic light emitting device including organic light emittingdevice units capable of emitting lights having different wavelengthssuch that light with desired colors can be emitted from the stackedorganic light emitting device. According to the above stacked organiclight emitting device, each organic light emitting device unit includestwo electrodes and a light emitting layer aligned between twoelectrodes. In addition, the electrodes are connected to the externalpower source, respectively, in such a manner that the organic lightemitting device units can be individually driven.

In addition, International Publication No. WO99/03158 discloses astacked organic light emitting device including organic light emittingdevice units capable of emitting lights having the same wavelength suchthat light with improved brightness can be emitted from the stackedorganic light emitting device. The stacked organic light emitting devicedisclosed in International Publication No. WO99/03158 has a structuresimilar to that of the stacked organic light emitting device disclosedin International Publication No. WO95/06400, except that the externalpower source is connected to both ends of the stacked organic lightemitting device. That is, external electrodes of the stacked organiclight emitting device are connected to the external power source andinternal electrodes of the stacked organic light emitting device aredisconnected from the external power source.

According to the above-mentioned stacked organic light emitting devices,internal electrodes interposed between the organic light emitting deviceunits include an internal anode in the form of a conductive thin filmelectrode made from indium-tin-oxide (ITO) or Au having a high workfunction, and an internal cathode in the form of a metal thin filmelectrode made from Al (4.28 eV), Ag (4.26 eV), or Ca (2.87 eV). Thus,two-layered internal electrodes, that is, the internal anode and theinternal cathode are aligned between the organic light emitting deviceunits of the stacked organic light emitting device while making contactwith each other. FIG. 1 shows such a stacked organic light emittingdevice including two-layered internal electrodes between the organiclight emitting device units.

However, according to the stacked organic light emitting device havingthe structure as shown in FIG. 1, since an ITO-based transparent oxideelectrode (internal anode) is formed on a metal thin film (internalcathode), to which electrons are injected, a physical bonding propertybetween the internal anode and the internal cathode is degraded, so thetwo-layered internal electrodes cannot be effectively formed. Inaddition, if the internal anode is made from ITO, a sputtering processmust be carried out due to the characteristics of the ITO. However, sucha sputtering process may increase kinetic energy of atoms (<KeV) ascompared with that of an evaporation process (<leV). For this reason, ifthe internal anode is formed through the sputtering process by usingITO, an organic semiconductor thin film already formed may besignificantly damaged (see, Journal of Applied Physics, vol. 86, no. 8,pp. 4607-4612, 1999).

In the meantime, European Patent Publication No. 1351558 A1 discloses astacked organic light emitting device including a single-layeredinternal electrode made from a single non-conductor thin film havingspecific resistance above 10⁵ Ωcm without forming two-layered internalelectrodes between stacked light emitting sections. The singlenon-conductor thin film is made from a material capable ofsimultaneously generating holes and electrons as an electric field isapplied to the stacked organic light emitting device in such a mannerthat the holes and electrons are injected into a hole transport layerand an electron transport layer, respectively. However, the singlenon-conductor thin film is very expensive and a method of forming thesingle non-conductor thin film is very difficult.

Accordingly, it is necessary to provide a stacked organic light emittingdevice, which can be easily fabricated without forming two-layeredinternal electrodes between stacked light emitting sections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a structure of a conventional stackedorganic light emitting device including two-layered internal electrodesformed between stacked light emitting sections.

FIG. 2 is a view illustrating a structure of a stacked organic lightemitting device including a single-layered internal electrode formedbetween stacked light emitting sections according to one embodiment ofthe present invention, wherein reference numerals 1 to 10 represent astacked organic light emitting device, a glass substrate, an externalanode, a first light emitting section, an internal electrode, a secondlight emitting section, an external cathode, first and second organiclight emitting units and an external power sources, respectively.

FIG. 3 is a view illustrating energy levels of organic material layersand internal electrode material formed in an organic light emittingdevice unit of a stacked organic light emitting device using asingle-layered internal electrode according to one embodiment of thepresent invention.

FIG. 4 is a graph illustrating UPS (ultraviolet photoelectronspectroscopy) data generated from a gold film and a HAT film formed onthe gold film with a thickness of about 20 nm according to ReferenceExample 1.

FIG. 5 is a graph illustrating a UV-VIS spectrum obtained from HATorganic material deposited on a glass surface according to ReferenceExample 1.

FIG. 6 is a view illustrating a structure of a single organic lightemitting device according to Comparative Example 1.

FIG. 7 is a view illustrating a structure of an organic light emittingdevice according to Comparative Example 2, in which the organic lightemitting device includes two-layered light emitting sections withoutforming an internal electrode therebetween.

FIG. 8 is a view illustrating a structure of a stacked organic lightemitting device according to Example 1, in which a single-layeredinternal electrode is formed between two-layered light emittingsections.

DISCLOSURE OF THE INVENTION

Inventors of the present invention have found that, in a stacked organiclight emitting device including an anode connected to an external powersource, a cathode connected to the external power source, at least twolight emitting sections including a light emitting layer aligned betweenthe anode and the cathode, and an internal electrode aligned between thelight emitting sections, if an organic material layer containing anorganic material having an electron affinity above 4 eV is formedbetween the light emitting layer of the light emitting section and theinternal electrode facing the anode connected to the external powersource in two internal electrode which make contact with the lightemitting section, it is possible to fabricate the internal electrode byusing a single-layered internal electrode made from one selected fromthe group consisting of a metal, alloys of the metal, and metal oxidesthereof, having a work function below 4.5 eV.

Accordingly, it is an object of the present invention to provide astacked organic light emitting device including a single-layeredinternal electrode aligned between stacked light emitting sections.

The present invention provides a stacked organic light emitting devicecomprising: an anode connected to an external power source; a cathodeconnected to the external power source; at least two light emittingsections aligned between the anode and the cathode, including a lightemitting layer; and an internal electrode aligned between the lightemitting sections, wherein the internal electrode is a single-layeredinternal electrode which is made from one selected from the groupconsisting of a metal, alloys of the metal, and metal oxides thereof,having a work function below 4.5 eV, each light emitting sectionincludes an organic material layer containing an organic material havingan electron affinity above 4 eV, and the organic material layer isformed between the light emitting layer of the light emitting sectionand the electrode facing the anode connected to the external powersource in two electrodes which make contact with the light emittingsection.

A structure of the stacked organic light emitting device according tothe present invention is shown in FIG. 2.

In addition, the present invention provides a display apparatusincluding the stacked organic light emitting device.

Definitions of terms used in the following description are as follows:

The light emitting section means an organic material layer unit, whichis aligned between an anode and a cathode in a single organic lightemitting device and includes a light emitting layer capable of emittinglight by receiving holes and electrons from the anode and the cathode,respectively. The organic material layer unit is different from theorganic light emitting device unit including the electrodes and thelight emitting section. The light emitting section can be provided inthe form of a single organic material layer acting as a light emittinglayer or in the form of a multiple organic material layer including ahole injection layer, a hole transport layer, a light emitting layer,and an electron transport layer.

The internal electrode means an electrode positioned between the lightemitting sections in the stacked organic light emitting device. Theinternal electrode is different from the external electrode positionedat an outermost portion of the stacked organic light emitting device.

The stacked organic light emitting device means a stacked structure ofsingle organic light emitting device units including an anode connectedto an external power source, a cathode connected to the external powersource, at least two light emitting sections aligned between the anodeand the cathode and including light emitting layers, and an internalelectrode formed between the stacked light emitting sections.

In addition, “HOMO” is an abbreviation of “the highest occupiedmolecular orbital” and “LUMO” is an abbreviation of “the lowestunoccupied molecular orbital”.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

In a conventional stacked organic light emitting device, two-layeredinternal electrodes including an internal anode and an internal cathode,which make contact with each other, are formed between the stacked lightemitting sections. In such a conventional stacked organic light emittingdevice, a material having a relatively high work function is used forthe internal anode for the purpose of hole injection and a materialhaving a relatively low work function is used for the internal cathodefor the purpose of electron injection.

However, inventors of the present invention have found that, if anorganic material layer containing an organic material having an electronaffinity above 4 eV is formed in the light emitting section between thelight emitting layer of the light emitting section and the electrodefacing the external anode in two electrodes which make contact with thelight emitting section, it is possible to fabricate the internalelectrode by using a single-layered internal electrode made from oneselected from the group consisting of a metal, alloys of the metal, andmetal oxides thereof, having a work function below 4.5 eV.

Hereinafter, an operational principle of the present invention will bedescribed with reference to FIG. 3.

In the present invention, a term “electron affinity” means a differencebetween a vacuum level and a LUMO energy level. The LUMO energy levelcan be calculated by adding an optical band gap to a HOMO energy level,wherein the HOMO energy level can be obtained by measuring ionizationpotential.

If an organic material layer containing an organic material having theelectron affinity above 4 eV is formed in the light emitting sectionbetween the light emitting layer of the light emitting section and theelectrode facing the external anode in two electrodes which make contactwith the light emitting section, there is no great difference betweenthe LUMO energy level of the organic material layer and the HOMO energylevel of the hole transport layer or the light emitting layer in thelight emitting section to which the organic material layer is adjacent(the HOMO energy level of the existing hole transport layer is set to5.0-6.0 eV). In addition, if the organic material layer has the electronaffinity above 4 eV, electrons in the HOMO energy level of the holetransport layer or the light emitting layer can be easily transferred tothe organic material layer. At this time, if the electrons in the HOMOenergy level of the hole transport layer or the light emitting layer areemitted, holes are generated at the very electron emitted site, i.e., inthe HOMO energy level of the hole transport layer or the light emittinglayer. The holes in the HOMO energy level may move to the light emittinglayer through the HOMO energy level, if necessary. Accordingly, when theorganic material layer is formed with an organic material having theelectron affinity above 4 eV, the organic material layer may act as ananode and/or a hole injection layer. In addition, the electrons shiftedinto the LUMO energy level of the organic material layer having theelectron affinity above 4 eV can move between molecules and have anelectric conductive characteristic, so that the electrons may movetoward an electrode facing the external anode due to an electricpotential between the external anode and the external cathode connectedto the external power source.

The reason for setting the electron affinity above 4 eV is to receiveelectrons from the hole transport layer or the light emitting layerpositioned adjacent to the organic material layer having the electronaffinity above 4 eV while injecting holes into the hole transport layeror the light emitting layer and to easily inject electrons into a metalinternal electrode.

Preferably, the organic material having a high electron affinity above 4eV ensures high carrier mobility. In this case, threshold voltage anddriving voltage of the device can be lowered.

According to the operational principle as mentioned above, the organicmaterial layer having the electron affinity above 4 eV, which is formedbetween the light emitting layer of the light emitting section and theelectrode facing the external anode in two electrodes which make contactwith the light emitting section, can act as an anode. Therefore,according to the stacked organic light emitting device of the presentinvention, it is possible to form a single-layered internal electrode byusing a metal, alloys of the metal, or metal oxides thereof, having awork function below 4.5 eV, which are known as materials for the cathodeof the organic light emitting device used for injecting electrons intothe organic material layer of the organic light emitting device whenexternal power is applied thereto, without forming a separate internalanode between the light emitting sections of the stacked organic lightemitting device.

An example of the organic material having the electron affinity above 4eV is shown in chemical formula 1.

In above chemical formula 1, R₁ to R₆ are one selected from the groupconsisting of hydrogen, halogen atom, nitrile (—CN), nitro (—NO₂),sulfonyl(—SO₂R), sulfoxide (—SOR), sulfonamide (—SO₂NR₂), sulfonate(—SO₃R), trifluoromethyl(—CF₃), ester (—COOR), amide (—CONHR or—CONRR′), substituted or unsubstituted linear or branched C₁-C₁₂ alkoxy,substituted or unsubstituted linear or branched C₁-C₁₂ alkyl,substituted or unsubstituted aromatic or non-aromatic heterocycliccompound, substituted or unsubstituted aryl, substituted orunsubstituted mono-arylamine or de-arylamine, and substituted orunsubstituted aralkyl. In addition, R and R′ are one selected from thegroup consisting of substituted or unsubstituted C₁-C₆₀ alkyl,substituted or unsubstituted aryl, and substituted or unsubstituted 5-7membered heterocyclic compound.

C₁-C₆₀ alkyl, aryl and heterocyclic compound of R and R′ can besubstituted with at least one functional group, which is one selectedfrom the group consisting of amine, amide, ether, and ester radicals.

In addition, aryl is one selected from the group consisting of phenyl,biphenyl, terphenyl, benzyl, naphthyl, anthracenyl, tetracenyl,pentacenyl, perylenyl, and coronenyl, which can be mono-substituted,poly-substituted or unsubstituted.

If an electron withdrawing functional group (hydrogen, halogen atom,nitrile (—CN), nitro (—NO₂), sulfonyl(—SO₂R), sulfoxide (—SOR),sulfonamide (—SO₂NR₂), sulfonate (—SO₃R), trifluoromethyl(—CF₃), ester(—COOR), amide (—CONHR or —CONRR′) is applied to R₁ to R₆, the electronsintroduced into π-orbital of a core structure shown in chemical formula1 are withdrawn by means of the electron withdrawing function group sothat the electrons are stabilized, i.e., re-localized, thereby levelingup the electron affinity, that is, a LUMO level may drop down.

According to the present invention, it is preferred to use —CN for R₁ toR₆.

Examples and synthesis methods for above chemical formula 1 aredisclosed in Korean Patent Application No. 10-2003-87159 in detail, thecontents of which are incorporated herein by reference.

Another example of the organic material having the electron affinityabove 4 eV includes 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F4TCNQ, LUMO energy level=5.24 eV), fluoride3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA), cyano PTCDA,naphthalenetetracarboxylicdianhydride (NTCDA), fluoride NTCDA and cyanoNTCDA.

According to the present invention, the single-layered internalelectrode is made from one selected from the group consisting of a metalhaving a work function below 4.5 eV, preferably below 4.3 eV, morepreferably in a range of 3.5 eV to 4.3 eV, alloys of the metal, andmetal oxides thereof. Since the single-layered internal electrode can bemade from materials having a low work function, electrons are effectiveinjected if the single-layered internal electrode is used as a cathodefor the organic light emitting device.

In addition, since the metal having the work function below 4.5 eV usedfor the internal electrode can be melted by means of heat, the internalelectrode can be formed through a thermal evaporation process instead ofa sputtering process. Thus, the stacked organic light emitting devicecan be prevented from being damaged and a process cost can be reduced.However, all metal alloy or metal oxide having the work function below4.5 eV is not adaptable to the thermal evaporation process. If the metalalloy or metal oxide has a high melting point, an e-beam evaporationprocess or a sputtering process is carried out.

The metal used for fabricating the single-layered internal electrodeincludes Al (4.28 eV), Ag (4.26 eV), Zn (4.33 eV), Nb (4.3 eV), Zr (4.05eV), Sn (4.42 eV), Ta (4.25 eV), V (4.3 eV), Hg (4.49 eV), Ga (4.2 eV),In (4.12 eV), Cd (4.22 eV), B (4.4 eV), Hf (3.9 eV), La (3.5 eV), Ti(4.3 eV), a Nd or Pd alloy thereof, Ca (2.87 eV), Mg (3.66 eV), Li (2.9eV), Na (2.75 eV), K (2.3 eV), Cs, (2.14 eV), or alloys thereof.However, the present invention is not limited to the above materials.Preferably, the single-layered internal electrode is made from oneselected from the group consisting of Al (4.28 eV), Ag (4.26 eV) andalloys thereof.

The thickness of the single-layered internal electrode can be adjustedby taking transmittance of light having a wavelength of a visible rayrange and electric conductivity into consideration. In detail, thesingle-layered internal electrode must represent superior transmittancefor the light in the visible ray range in order to allow the lightgenerated from the light emitting device to be easily emitted to anexterior. To this end, it is preferred for the internal electrode tohave a thin thickness as possible. However, a thin film made from ametal may represent low electric conductivity even if the metal hassuperior electric conductivity. Therefore, it is necessary to adjust thethickness of the internal electrode in such a manner that the internalelectrode has superior electric conductivity while representing superiortransmittance for the light. According to the present invention, thethickness of the internal electrode is preferably adjusted in a range ofabout 1 to 100 Å.

According to the present invention, if the light emitting sectionaligned adjacent to the external anode includes the organic materiallayer containing the organic material having the electron affinity above4 eV between the light emitting layer of the light emitting section andthe external anode, the external anode can be fabricated by using amaterial having a relatively low work function as used for the internalelectrode, as well as a material having a relatively high work functionused for the anode of the conventional organic light emitting device.Therefore, the external anode can be fabricated not only using materialsused for the internal electrode, but also using a metal having a highwork function, such as V, Cr, Cu, Zn, Au or an alloy thereof, metaloxides, such as Zn oxide, In oxide, ITO (indium-tin-oxide) or In—Znoxide, a combination of a metal and oxide, such as ZnO:Al or SnO₂:Sb,and conductive polymer, such as poly(3-methylthiophene),poly[3,4-(ethylene-1,2-dioxy)thiophene] (PEDT), polypyrrole, orpolyaniline. However, the present invention is not limited to the abovematerials.

According to the present invention, the external cathode can befabricated by using a material having a relatively low work function insuch a manner that electrons can be easily injected into the organicmaterial layer. In detail, the external cathode can be fabricated notonly using materials used for the internal electrode, but also using ametal, such as Mg, Ca, Na, Yt, Li, Gd, Pb or an alloy thereof, and amulti-layered material, such as LiF/Al or LiO₂/Al. However, the presentinvention is not limited to the above materials.

The stacked organic light emitting device according to one embodiment ofthe present invention can be fabricated through the following processes.The structure of the stacked organic light emitting device fabricatedaccording to one embodiment of the present invention is shown in FIG. 2.As shown in FIG. 2, an anode 3 made from one of the above-mentionedanode materials is formed on a transparent glass substrate 2. Then, afirst light emitting section 4 including a light emitting layer isformed on the anode 3. The light emitting section 4 can be fabricatedwith a multi-layered structure or a single-layered structure. Themulti-layered structure may include a hole injection layer, a holetransport layer, a light emitting layer and an electron transport layer.According to the present invention, the light emitting section includesan organic material layer containing an organic material having anelectron affinity above 4 eV formed between the light emitting layer ofthe light emitting section and the electrode facing the external anodein two electrodes which make contact with the light emitting section. Asmentioned above, the organic material layer containing the organicmaterial having the electron affinity above 4 eV plays a role of a holeinjection layer or a hole transport layer. In addition, the organicmaterial layer can be used for the purpose of hole injection and holetransportation. Then, an internal electrode 5 having a thickness in arange of about a few Å to tens of Å is formed on the light emittingsection 4 by using one of the above-mentioned materials. In addition, asecond light emitting section 6 is formed on the internal electrode inthe same manner as the first light emitting section 4. At this time, theinternal electrode and the light emitting sections can be repeatedlyformed by several times, if necessary. Finally, a cathode 7 is formed onthe second light emitting section 6 by using one of one of theabove-mentioned cathode materials. Herein, the electrode and the lightemitting section including an organic material layer can be formedthrough a conventional technology generally known in the art.

The stacked light emitting sections are formed in the same structure byusing the same material. However, it is also possible to form thestacked light emitting sections in different structures or by usingdifferent materials.

The stacked organic light emitting device of the present invention canbe fabricated in the form of a top emission type organic light emittingdevice, a bottom emission type organic light emitting device, or a dualemission type organic light emitting device according to materials usedfor the stacked organic light emitting device.

In addition, a display apparatus including the stacked organic lightemitting device of the present invention can be fabricated through aconventional method generally known in the art.

Since the organic light emitting device units are connected to eachother in series in the stacked organic light emitting device of thepresent invention, density of photons generated from the light emittinglayer of each light emitting section may increase under the same currentdensity, so that emission efficiency and brightness of light can beimproved proportionally to the number of stacked light emittingsections.

In addition, according to the stacked organic light emitting device ofthe present invention, the light emitting section is fabricated in sucha manner that it has a light emission spectrum of a color selected fromred, green, blue, and a combination thereof. Thus, a light emittingdevice capable of emitting light having a white color or a desired colorcan be obtained by stacking the light emitting sections.

In the meantime, if the thickness of the light emitting section providedin the stacked organic light emitting device according to the presentinvention is properly adjusted, at least two overlapped optical peakscan be generated due to micro-cavity effect, so that a white light of awide range can be produced, although it does not precisely match withthe white light defined in a CRI (color rendition index). That is, if aninternal electrode having a high reflective property is used, some oflight generated from the light emitting section is reflected from theinternal electrode without being emitted to the exterior, therebycausing light interference in the stacked organic light emitting device.In this state, if the thickness of the organic material layer providedin the light emitting section is properly adjusted, a light emittingspectrum is changed, which is called “micro-cavity effect”.

In addition, since the present invention employs a single-layeredinternal electrode, the problem occurring in the conventional stackedorganic light emitting device in relation to two-layered internalelectrodes, such as poor physical bonding force between internalelectrodes, may not happen. Conventionally, the organic light emittingdevice is easily damaged when fabricating the internal anode through thesputtering process. However, the single-layered internal electrode ofthe present invention can be made from a material adaptable for theevaporation process, so the stacked organic light emitting device of thepresent invention can be prevented from being damaged, thereby improvingstability of the stacked organic light emitting device. In addition,since the present invention employs the single-layered internalelectrode, the present invention represents advantages in view of themanufacturing cost and manufacturing process as compared with theconventional art employing two-layered internal electrodes, which mustbe bonded to each other.

Hereinafter, embodiments of the present invention will be described indetail. It should be noted that the following embodiments are forillustrative purposes only and are not intended to limit the scope ofthe present invention.

Embodiments Reference Example 1 HOMO and LUMO Levels of HAT (USP andUV-VIS Absorption Schemes)

In order to detect the HOMO level of Hexanitrile hexaazatriphenylene(HAT), a UPS (Ultraviolet photoelectron spectroscopy) scheme was used.According to the UPS scheme, kinetic energy of the electrons emittedfrom a sample was analyzed while radiating vacuum UV line (21.20 eV)generated from a helium lamp onto the sample under an ultra high vacuumstate (<10⁻⁸ Torr), thereby detecting the work function of the metalsand ionization energy of organic materials, that is the HOMO level andthe Fermi energy level. When the vacuum UV line (21.20 eV) radiates ontothe sample, the kinetic energy of the electrons emitted from the samplecorresponds to a difference between vacuum UV energy (21.20 eV) andelectron binding energy of the sample to be measured. Accordingly, thebinding energy distribution of materials in the sample can be detectedby analyzing the distribution of the kinetic energy of the electronsemitted from the sample. If the kinetic energy of the electronrepresents a maximum value, the binding energy of the sample representsa minimum value. Thus, the work function (Fermi energy level) and theHOMO level of the sample can be determined.

In the reference example 1, after obtaining the work function of gold byusing a gold film, the kinetic energy of the electrons emitted from aHAT film was analyzed while depositing the HAT film on the gold film, inorder to obtain the HOMO level of the HAT. FIG. 4 shows UPS datagenerated from the gold film and the HAT film formed on the gold filmwith a thickness of about 20 nm. Hereinafter, the description will bemade with reference to terminology disclosed in a document (H. Ishii, etal, Advanced Materials, 11, 605-625 (1999)). Binding energy shown in anx-axis of FIG. 7 was calculated based on the work function measured fromthe metal film. That is, the work function of gold in reference example1 can be obtained by subtracting a maximum value (15.92 eV) of thebinding energy from vacuum UV line (21.20 eV). According to referenceexample 1, the work function of gold was 5.28 eV. In addition, the HOMOlevel of the HAT can be obtained by subtracting a difference valuebetween the maximum value (15.19 eV) and the minimum value (3.79) of thebinding energy from the vacuum UV line (21.20 eV) radiated on the HATdeposited on the gold film. According to reference example 1, the HOMOlevel of the HAT was 9.80 eV and the Fermi level was 6.02 eV.

An UV-VIS spectrum as shown in FIG. 5 was obtained by using an organicmaterial formed by depositing the HAT on a surface of glass. Inaddition, a band gap of 3.26 eV was obtained by analyzing an absorptionedge. Thus, it is understood that the LUMO of the HAT is below 6.54 eV.The above value may vary according to exciton binding energy of the HATfilm. That is, since 6.54 eV is larger than the Fermi level (6.02 eV) ofthe HAT film, the exciton binding energy of the HAT film must exceed0.52 eV in order to set the LUMO level lower than the Fermi level. Ingeneral, the exciton binding energy of the organic material is 0.52 eV(maximum value thereof is lower than 1 eV), so the LUMO level of the HATis in the range of 5.54 to 6.02 eV.

Comparative Example 1

A single organic light emitting device including a single organic lightemitting section is fabricated as follows:

(1) Forming of Anode

A transparent anode having a thickness of about 1500 Å was formed on atransparent glass substrate by using ITO (indium-tin-oxide) through asputtering process. Then, the transparent anode was subject to a plasmaprocess by using forming gas, which was obtained by adding 4% of H₂ toAr.

(2) Forming of Light Emitting Section

Hexanitrile hexaazatriphenylene (HAT) as shown in chemical formula 1awas deposited on the transparent anode through a vacuum depositionprocess, thereby forming a hole injection layer having a thickness ofabout 500 Å.

After that, NPB was deposited on the hole injection layer through avacuum deposition process, thereby forming a hole transport layer havinga thickness of about 400 Å. Then, Alq3 doped with 1% of dopant (C545T)available from Kodak company was deposited on the hole transport layerthrough a vacuum deposition process, thereby forming a light emittinglayer having a thickness of about 300 Å. After that, a compound as shownin chemical formula 2, which is disclosed in Korean Patent ApplicationNo. 10-2002-3025, was deposited on the light emitting layer through avacuum deposition process, thereby forming an electron transport layerhaving a thickness of about 200 Å.

(3) Forming of Cathode

LiF and Al were sequentially deposited on the electron transport layerthrough an evaporation process, thereby forming a cathode.

A structure of an organic light emitting device fabricated through theabove processes is shown in FIG. 6.

Comparative Example 2

According to comparative example 2, the organic light emitting devicewas fabricated by performing the process identical to that ofcomparative example 1, except that the process for forming the lightemitting section was repeated by two times. Therefore, the stackedorganic light emitting device included two light emitting sectionswithout forming an internal electrode (an internal cathode (Al) and aninternal anode (ITO)). A structure of the stacked organic light emittingdevice fabricated through the above processes is shown in FIG. 7.

Comparative Example 3

According to comparative example 3, the organic light emitting devicewas fabricated by performing the process identical to that ofcomparative example 1, except that the process for forming the organiclight emitting section was repeated by two times, in which a singlelayered internal electrode having a thickness of about 60 Å was formedby using Al after forming a first light emitting section, and then, asecond light emitting section was formed without forming a HAT organicmaterial layer. Thus, the organic light emitting device of comparativeexample 3 included two light emitting sections and an internal electrodemade from AL interposed between two light emitting sections, in whichthe internal electrode did not include an internal anode (ITO) and theHAT organic material layer was omitted from the second light emittingsection. A structure of the stacked organic light emitting devicefabricated through the above processes is shown in FIG. 8.

Example 1

According to example 1, the organic light emitting device was fabricatedby performing the process identical to that of comparative example 1,except that a single layered internal electrode having a thickness ofabout 60 Å was formed by using Al after forming a first light emittingsection, and then, a second light emitting section was formed. Thus, theorganic light emitting device of example 1 included two light emittingsections and an internal electrode made from AL interposed between twolight emitting sections, in which the second light emitting section hada HAT organic material layer. A structure of the stacked organic lightemitting device fabricated through the above processes is shown in FIG.8.

Test Result

In a case of the single organic light emitting device of comparativeexample 1, current density of 10 mA/cm² was represented under appliedvoltage of 3.9V. At this time, a light emitting efficiency was 7.9 cd/A,and brightness was 790 cd/m². In a case of the stacked organic lightemitting device of comparative example 2, which does not include theinternal electrode, current density of 10 mA/cm² was represented underapplied voltage of 8.7V. At this time, a light emitting efficiency was7.4 cd/A, and brightness was 742 cd/m². In a case of the stacked organiclight emitting device of comparative example 3, in which the secondlight emitting section did not include the HAT organic material layer,current density of 10 mA/cm² was represented under applied voltage of16.5V. At this time, a light emitting efficiency was 5 cd/A, andbrightness was 500 cd/m². In addition, in a case of the stacked organiclight emitting device of example 1, similarly to comparative example 2,current density of 10 mA/cm² was represented under applied voltage of8.7V. At this time, a light emitting efficiency was 13.8 cd/A, andbrightness was 1380 cd/m². Table 1 shows the above test result.

TABLE 1 Light Current Applied emitting density voltage efficiencyBrightness Comparative 10 mA/cm² 3.9 V 7.9 cd/A 790 cd/m² example 1Comparative 10 mA/cm² 8.7 V 7.4 cd/A 742 cd/m² example 2 Comparative 10mA/cm² 16.5 V   5 cd/A 500 cd/m² example 3 Example 1 10 mA/cm² 8.7 V13.8 cd/A  1380 cd/m² 

In the case of the organic light emitting device of comparative example2, in which the thickness of the stacked light emitting sections wasenlarged by two times as compared with that of the comparative example 1because the light emitting sections were stacked without forming theinternal electrode therebetween, although applied voltage thereof wasincreased by approximately two times as compared with that ofcomparative example 1 in order to obtain the same current densitythereof as that of comparative example 1, the light emitting efficiencyand brightness thereof were similar to those of comparative example 1.It can be understood from the above result that, if the thickness of theorganic material layer of the organic light emitting device is simplyenlarged, the driving voltage must be increased in order to obtain thesame current density, light emitting efficiency and brightness.

The light emitting efficiency and brightness of the stacked organiclight emitting device according to example 1 were increased by two timesas compared with those of the single light emitting device ofcomparative example 1 and the stacked organic light emitting device ofcomparative example 1, which did not includes the internal electrode. Inaddition, when comparing example 1 with comparative example 3, it can beunderstood that the driving voltage is lowered and the light emittingefficiency and brightness are improved if the organic material layerhaving the electron affinity above 4 eV is interposed between theinternal electrode and the light emitting layer, even if the internalelectrode includes only the internal cathode without the internal anode.In addition, it can be understood from the above result that the singlelayered internal electrode interposed between the two light emittingsections and the organic material layer having the electron affinityabove 4 eV may act as the internal anode and the internal cathode withrespect to each of the stacked light emitting sections. That is, thesingle layered internal electrode and the organic material layer havingthe electron affinity above 4 eV may act as the hole injection layer andthe electron injection layer.

INDUSTRIAL APPLICABILITY

As described above, according to the stacked organic light emittingdevice of the present invention, the light emitting efficiency andbright are improved proportionally to the number of stacked lightemitting sections, and desired light can be emitted according towavelengths of the light emitting sections. In addition, since thestacked organic light emitting device of the present invention includesthe single layered internal electrode, the stacked organic lightemitting device of the present invention can be easily fabricated at alow cost as compared with the conventional organic light emitting devicehaving the two-layered internal electrodes. Furthermore, since it is notnecessary to use materials adaptable for the sputtering process whenforming the internal electrode, the stability of the stacked organiclight emitting device according to the present invention can beimproved.

The invention claimed is:
 1. A stacked organic light emitting devicecomprising: an anode connected to an external power source; a cathodeconnected to the external power source; at least two light emittingsections aligned between the anode and the cathode, each light emittingsection including a light emitting layer; and an internal electrodealigned between the light emitting sections, wherein the internalelectrode is a single-layered internal electrode consisting of a metalor a metal alloy, having a work function below 4.5 eV, wherein one ofthe at least two light emitting sections includes an organic materiallayer containing an organic material having an electron affinity above 4eV, and wherein the organic material layer is formed between the lightemitting layer of the one of the at least two light emitting sectionsand the internal electrode.
 2. The stacked organic light emitting deviceas claimed in claim 1, wherein the organic material having the electronaffinity above 4 eV is a compound having a chemical formula as shownbelow,

wherein, R₁ to R₆ are one selected from the group consisting ofhydrogen, halogen atom, nitrile (—CN), nitro (—NO₂), sulfonyl (—SO₂R),sulfoxide (—SOR), sulfonamide (—SO₂NR₂), sulfonate (—SO₃R),trifluoromethyl (—CF₃), ester (—COOR), amide (—CONHR or —CONRR′),substituted or unsubstituted linear or branched C₁-C₁₂ alkoxy,substituted or unsubstituted linear or branched C₁-C₁₂ alkyl,substituted or unsubstituted aromatic or non-aromatic heterocycliccompound, substituted or unsubstituted aryl, substituted orunsubstituted mono-arylamine or de-arylamine, and substituted orunsubstituted aralkyl, and R and R′ are one selected from the groupconsisting of substituted or unsubstituted C₁-C₆₀ alkyl, substituted orunsubstituted aryl, and substituted or unsubstituted 5-7 memberedheterocyclic compound.
 3. The stacked organic light emitting device asclaimed in claim 2, wherein R₁ to R₆ are nitrile (—CN).
 4. The stackedorganic light emitting device as claimed in claim 1, wherein theinternal electrode is made from at least one selected from the groupconsisting of Al (4.28 eV), Ag (4.26 eV), Zn (4.33 eV), Nb (4.3 eV), Zr(4.05 eV), Sn (4.42 eV), Ta (4.25 eV), V (4.3 eV), Hg (4.49 eV), Ga (4.2eV), In (4.12 eV), Cd (4.22 eV), B (4.4 eV), Hf (3.9 eV), La (3.5 eV),Ti (4.3 eV), Ca (2.87 eV), Mg (3.66 eV), Li (2.9 eV), Na (2.75 eV), K(2.3 eV), Cs, (2.14 eV), and alloys thereof.
 5. The stacked organiclight emitting device as claimed in claim 1, wherein each of the stackedlight emitting sections has a light emission spectrum of a colorselected from red, green, blue, and combination thereof, so that thestacked organic light emitting device is able to emit white light. 6.The stacked organic light emitting device as claimed in claim 1, whereinthe stacked organic light emitting device is able to emit a white lightof a wide range due to a micro-cavity effect.
 7. The stacked organiclight emitting device as claimed in claim 1, wherein the organicmaterial having the electron affinity above 4 eV is one selected fromthe group consisting of2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, fluoride3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA), cyano PTCDA,naphthalenetetracarboxylicdianhydride (NTCDA), fluoride NTCDA and cyanoNTCDA.
 8. The stacked organic light emitting device as claimed in claim1 further comprising: at least three light emitting sections, and atleast two internal electrodes.
 9. The stacked organic light emittingdevice of claim 8, wherein two of the at least three light emittingsections each includes a light emitting layer and an organic materiallayer, the organic material layer containing an organic material havingan electron affinity above 4 eV, and wherein each organic material layeris formed between the light emitting layer of the corresponding lightemitting section and one of the at least two internal electrodes. 10.The stacked organic light emitting device of claim 1, wherein all butone of the at least two light emitting sections each includes a lightemitting layer and an organic material layer containing an organicmaterial having an electron affinity above 4 eV, and wherein eachorganic material layer is formed between the light emitting layer of thecorresponding light emitting section and an internal electrode.
 11. Thestacked organic light emitting device of claim 1, wherein another of theat least two light emitting sections is located between the internalelectrode and anode.
 12. A display apparatus comprising a stackedorganic light emitting device as claimed in claim
 1. 13. The displayapparatus as claimed in claim 12, wherein the internal electrode is madefrom at least one selected from the group consisting of AL (4.28 eV), Ag(4.26 eV), Zn (4.33 eV), Nb (4.3 eV), Zr (4.05 eV), Sn (4.42 Ev), Ta(4.25 eV) V (4.3 eV), Hg (4.49 eV), Ga (4.2 eV), In (4.12 eV) Cd (4.22eC), B (4.4 eV), Hf (3.9 eV), La (3.5 eV), Ti (4.3 eV), Ca (2.87 eV), Mg(3.66 eV), Li (2.9 eV), Na (2.75 eV), K (2.3 eV), Cs, (2.14 eV), andalloys thereof.
 14. The display apparatus as claimed in claim 12,wherein each of the stacked light emitting sections has a light emissionspectrum of a color selected from red, green, blue, and combinationthereof, so that the stacked organic light emitting device is able toemit white light.
 15. The display apparatus as claim in claim 12,wherein the stacked organic light emitting device is able to emit awhite light of a wide range due to a micro-cavity effect.
 16. Thedisplay apparatus as claimed in claim 12, wherein the organic materialhaving the electron affinity above 4 eV is a compound having a chemicalformula as shown below,

wherein, R₁ to R₆ are one selected from the group consisting ofhydrogen, halogen atom, nitrile (—CN), nitro (—NO₂), sulfonyl (—SO₂R),sulfoxide (—SOR), sulfonamide (—SO₂NR₂), sulfonate (—SO₃R),trifluoromethyl (—CF₃), ester (—COOR), amide (—CONHR or —CONRR′),substituted or unsubstituted linear or branched C₁-C₁₂ alkoxy,substituted or unsubstituted linear or branched C₁-C₁₂ alkyl,substituted or unsubstituted aromatic or non-aromatic heterocycliccompound, substituted or unsubstituted aryl, substituted orunsubstituted mono-arylamine or de-arylamine, and substituted orunsubstituted aralkyl, and R and R′ are one selected from the groupconsisting of substituted or unsubstituted C₁-C₆₀ alkyl, substituted orunsubstituted aryl, and substituted or unsubstituted 5-7 memberedheterocyclic compound.
 17. The display of apparatus as claimed in claim16, wherein R₁ to R₆ are nitrile (—CN).
 18. The display apparatus asclaimed in claim 12, wherein the organic material having the electronaffinity above 4 eV is one selected from the group consisting of2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, fluoride3,4,9,10-perylenetetracarboxylidianhydride (PTCDA), cyano PTCDA,naphthalenetetracarboxylicdianhydride (NTCDA), fluoride NTCDA and cyanoNTCDA.